The advent of inexpensive integrated circuitry has facilitated the conversion of analog voice and data signals into digitized samples, generally in a PCM mode, as well as the reconversion of such samples into analog signals. Converters of this type can therefore now be installed directly in a telephone subscriber's apparatus so as to allow the transmission of digital signals between a subscriber station and a central office or exchange, with resulting improvement in signal quality and a more extended use of a telephone line for additional services such as data transmission.
A convenient path for the bidirectional transmission of digital signals would be a 4-wire line, with respective wire pairs available for incoming and outgoing signals. Since, however, in existing telephone networks an exchange is generally linked with associated subscriber stations by 2-wire lines, their replacement with 4-wire transmission links would be prohibitively expensive.
When a 2-wire line is to be used for bidirectional transmission of digitized signals, incoming and outgoing samples may be alternately sent over the line in the form of respective binary sequences separated by a guard interval within a PCM frame. This would require an approximate doubling of the bit rate and a corresponding increase in bandwidth if the number of intercommunicating channels is to be preserved. Since the usual subscriber lines can operate only with limited bandwidth, this method is also not very practical.
Conventional duplexing terminals, using hybrid coils, enable the simultaneous transmission of analog signals in two directions over a single 2-wire loop. With digitized samples, however, the effect of echoes of outgoing signals upon incoming signals is more serious, such echoes being almost unavoidably caused by imperfect impedance matching between the line and the balancing load of the hybrid coil as well as by reflections at parallel stubs adjoining the line. Prior proposals for minimizing these echoes call for a repetitive evaluation of the noise during a given bit period for the generation of corrective signals by relatively costly recursive filtering.
In commonly owned application Ser. No. 193,009, now U.S. Pat. No. 4,393,494, filed Oct. 2, 1980 by Piero Belforte et al, there has been disclosed a transceiver for full-duplex transmission of digital signals over a line with not more than two wires. While that transceiver constitutes a significant improvement over the conventional hybrid-coil termination, its implementation is still somewhat expensive if echo noises are to be fully suppressed.